The primary evidence of reduced frequency selectivity in listeners with sensorineural hearing loss comes from masking studies where these listeners are found to have abnormally high thresholds and abnormally poor speech perception. This proposal outlines a series of studies designed to test the hypothesis that the effect of hearing loss combines with the effect of an external masker in the same way that the effect of one external masker combines with the effect of another. Recent research suggests that most combinations of external maskers produce more masking in listeners with normal hearing than would be predicted from the effects of the individual maskers and that the rules governing effects of combined maskers differ for different types of maskers. Recent models of additivity of masking provide a framework for integrating these diverse results which could be expanded to include effects of sensorineural hearing loss. The proposed work includes further development of models of additivity, further studies of additivity in both normal and impaired listeners, and studies to extend this approach to encompass loudness growth and speech perception. Hearing loss and masking are clearly not equivalent. Learning more about precisely how they differ will lead to a better understanding of both processes and of how they interact with one another.